1. Field of the Invention
The invention relates to a printed web-type material, in particular a lid for a container for dairy products, comprising a film of plastics, paper or metal on the visible surface.
2. Background Art
Producing the overprint on lids of the generic type conventionally takes place by flexo or gravure printing. These printing methods are of poor efficiency in the manufacture of minor orders. Any need for identity of quality of the closing film or lid with the final product that will be commercialized, for instance for fairs or internal product presentation, will require investments in costly impression cylinders and machine setting. Alternatively, samples have to be prepared using other printing methods, which will however not be comparable to the originals made on a printing machine. If modifications become necessary after such a product presentation and impression cylinders have already been made, modified impression cylinders have to be prepared anew, which is also rather costly.
In this regard, U.S. patent application Ser. No. 09/512,066 teaches to arrange the digital-printer-made overprint between two layers, one of them being a low-temperature hot-sealing varnish layer. Using a digital printer meets the requirements which rival per se: lids of the generic type are customarily sealed at temperatures ranging between 180xc2x0 C. and 250xc2x0 C., whereas the digital printing inks currently available are stable only up to 100xc2x0 C. and the usual hot-sealing varnishes really start sealing at 160xc2x0 C. The prior art method of embedding the digital printing inks in between two layers helps protect the printing inks, and using a low-temperature hot-sealing varnish layer prevents them from being damaged by elevated temperatures.
The lack of temperature resistance of the printing inks may give rise to problems also for applications of a different kind, for example when decoration films are applied to chipboards by means of hot-melt adhesives as for instance in the field of furniture making.
It is an object of the invention, while keeping the advantages of the prior art arrangement, to embody a printed material, in particular a lid, which can be equipped and worked using any hot-sealing varnishes, and more generally to obtain temperature stability in a printed web of material.
According to the invention, this object is attained in that a thermoplastic primer, which is needed for applying of the overprint, and the printing ink itself are covered by a varnish of identical or similar composition so that part of the curing agent, which is applied separately or is available in the varnish, migrates into the printing ink or the thermoplastic primer, resulting in cross-linking.
In this way, a per se thermoplastic system of a softening temperature of approximately 90xc2x0 C. will pass into a heat-resistant system of a temperature resistance of approximately 280xc2x0 C. The corresponding reaction is time-dependent and terminated after approximately four days at ambient temperature. Due to the heat resistance thus obtained, it is possible to use any conventional hot-sealing varnishes for closing a container by means of a lid of the generic type or to ensure further workability by means of a hot-melt adhesive.
Preferably it is provided that the termoplastic primer or the varnish is an ethylene acrylate copolymer dispersion or a blend thereof with polyester, PVAC or polyamide.
Favorably, a polyfunctional aziridine can be used as a curing agent.
The cross-linked product according to the invention is in particular a polycondensate.
The embodiment according to the invention is suitable in particular in combination with a digital overprint that is produced by electrostatically generated image buildup, proceeding from a liquid thermoplastic toner and by transfer onto an intermediate. This means that there is no direct contact to the material to be printed, which is why even aluminum foil can be printed.
Such a printing method is also termed indigo printing. In addition to the color pigments, the printing inks used contain charge molecules which can be aligned in the electric field. For printing, a photosensitive coating is charged electrostatically to approximately 800 V and then laser-irradiated. The electric charge drops to approximately 100 V at the exposed spots. This is where the color particles of the printing ink accumulate in the case of full-face coloring of the coating. The non-exposed spots do not accept any color because the charge is too high. The image thus printed is transferred onto an intermediate, a so-called blanket. The blanket temperature is approximately 130xc2x0 C. This is where the flotation oil contained in the ink evaporates, and the entire printed image polymerizes into a thermoplastic layer. This layer can be transferred onto paper without a primer, transfer onto aluminum foils or the like is possible only by using a primer with which to equip the web-type material. The softening point of the primer must be distinctly below the blanket temperature of 130xc2x0 C. Another important property of the primer is a high hot tack which confers sufficient adhesion to the material to be printed as well as to the layer of printing ink, ensuring impeccable transfer.
By alternative, printing may take place using liquid ink and a print head or by means of a solid thermoplastic toner.